The reason nobody is answering is that nobody knows.
I think it's irresponsible of you to just say "Replace the wiper motor!"
As if that didn't cross my mind and I didn't talk to the local Mercedes Guru Heinz about this.
He said in a very quiet German accent: "Somethings f@cking wet in there and it could take me hours to find it"
At $90 an hour I'm sure as hell gonna try to solve it myself first.

So, you stated your opinion i read it and I disagree, now please drop it and leave this open for someone who has another idea.

Don't knock the black silicone, it rejuvenates door seals, fixes cracks in the dash and spread nice on toast.